This application is based on Japanese patent application No. 2010-294504 filed on Dec. 29, 2010, the contents of which are hereby incorporated by reference.
Recent years have seen the widespread use of image forming apparatuses that include various functions such as copying, PC printing, scanning, faxing, and serving as a file server. Such image forming apparatuses are called a “multifunction device” or an “MFP” (Multi-Functional Peripheral).
PC printing is a function for receiving image data from a personal computer and printing an image on a sheet.
Also, applications for performing rendering with a personal computer have been distributed in recent years.
Such applications are called “rendering software”. Some rendering software includes a function for displaying a transparent image on a display.
A “transparent image” is characteristic in that an image of an object that is behind the transparent image can be seen through the transparent image.
Specifically, as shown in FIG. 4A for example, a transparent image 40a is overlaid on the left half of a background image 40b. As shown in FIG. 4B, the portion of the background image 40b that is overlapped by the transparent image 40a can be seen through the transparent image 40a. However, when a non-transparent image 40c, which is not a transparent image, is overlaid on the right half of the background image 40b, the background image 40b cannot be seen through the non-transparent image 40c. The higher the transparency of the transparent image is, the more visible the background image that is overlapped by the transparent image is.
A transparent image displayed by a personal computer can be printed on a sheet by an image forming apparatus. Before the transparent image is printed, pixel thinning-out processing is performed in accordance with the level of transparency as shown in FIGS. 5B and 5C. The image behind the transparent image is then printed in the positions of the pixels that were removed in the thinning-out processing. Accordingly, the background image can be seen through the transparent image.
Also, technology for detecting characters such as letters and numbers in an image has been put into practical use. Furthermore, methods for precisely detecting characters have also been proposed. For example, methods such as the following have been proposed.
A digital image is divided into multiple blocks, a contrast amount relating to the pixel values of the pixels included in a block is obtained for each block, a pixel value bimodality evaluation value relating to a histogram of the pixel values of the pixels included in a block is obtained for each block, a contrast threshold value is obtained based on the contrast amounts, a bimodality threshold value is obtained based on the pixel value bimodality evaluation values, and the blocks are classified as text blocks or non-text blocks. In this classifying, a block is classified into a text block if the contrast amount and the pixel value bimodality evaluation value thereof satisfy a first criterion that is based on the contrast threshold value and the bimodality threshold value, and a block is classified into a non-text block if the first criterion is not satisfied (Patent Literature 1).